Oscillator circuits



May 10, 19'49.- w. E. BRADLEY ET AL 2,469,811

OSCILLATOR CIRCUITS Filed Sept. 17, 1946 EG- Z. INVENTOR) Patented May 10, 1949 UNITED STATES PATENT OFFICE 12 Claims.

This invention relates to vacuum tube oscillators. More specifically it relates to anoscillator which is adapted selectively to generate electrical wave signals at either of two slightly different frequencies, the desired frequency at anyparticular instant of time being readily determinaole, in response to a control signal suppliedto the oscillator from an external source.

Frequently in the communications andradar art it is desired selectively to provide signals of two slightly and precisely differing frequencies. Although it is quite feasible to do this by using any one of a number of conventional means, the amount of equipment requiredand the complexity of the circuits may prove to be objectionable. Accordingly, it is the primary object of-ourinvention to provide a simple, convenient and'reliable oscillator for yielding the desired result while employing a minimum of equipment. The oscillator hereinafter disclosed employs but a-pair of vacuum tube sections, or their'equivalentpand a single tank circuit, plus afew additional components. It is extremely simple in operationand readily controllable, for example in response to a signal of alternating rectangular waveform, for

operation selectively at either of two differing frequencies. Also it is eminently adapted for-incorporation into other circuits in which,1w-hile performing its primary function as a generator,

it may also be made to cooperate in other waysto bring about additional desired'results.

Thus, for example, as set forth hereinafter, the two tubes of the oscillator circuit. may bemade to function alternately as superregenerative amplifiers at two slightly different frequencies. Such -J an arrangement has particular application ina detector of angular-velocity modulated carrier wave signals, one form of which is hereinafter shown and described.

The principle of our invention and the.con--,

tion of the circuit of Figure 1 to a superregenera-- tive detector of angular-velocity modulated carrier wave signals.

Referring to Figure 1, the oscillator circuit shown comprises a pair of triode vacuum tubes I and 2 connected so as to form, .in'cooperation with a common tank circuit 3, separate oscillator circuits which are, respectively, of the wellknown Hartley vandColpitts forms. Thus. tank circuitinductor i is connected between the grid of tube i and ground, and the cathode of tube. I

is connected to a tap on inductor 4 so that the lower portion of the latter is included in the cathode circuit of the tube and provides the feedback necessary for oscillation. Tube I, in cooperation;withtank circuit 3, thereforeforms a conventional Hartley circuit. On the other hand, tank circuit'condenser 5 is connected between thegrid and cathode of tube 2, and condenser 6, also part of tank circuit 3, is connected between the cathode of tube 2 and ground as in a conventional Colpitts circuit. Conventional grid resistors I and 8 and gridcondensers Sand IIlarelikewise provided for both tubes i and 2.

By application of a suitable controlling signal to appropriate elements of either or both of tubes I and 2, one tube may be cut off during prescribed intervals, while the other tube is conductive, and .vice .versa. Thus each tube maybe permitted to oscillate separately and independ- .ently of the other during predetermined intervals. The mode of. operation of the circuit incorporating tube I is that of a conventional Hartley and, of 'the circuit incorporating tube 2, th at of aconventional Colpitts. S0 generally understood is the operation of, these circuits, it is not deemed necessary to devote further discussion to tion, each of the separate. oscillators is topperate at aslightly different frequency and, to this .end, resistors I2 and I3 are connectedrespectively fromthe cathodes of tubes I and, 2 to ground. This willeffect, in the instance ofttube I, a slight decrease in its frequency of oscillation, and, inthe .instanceof tube 2 a slight increase. In orderthat each circuit shall continue to be capable of sustaining oscillations, it is essential that the magnitudes of resistors I'Zand It be not ,too small. -=Furthermore, by appropriate selection of the values of these resistors, it is possible gto achieve a maximum, or any desired smaller,

. choosingthe'values of resistors I2 and 13 will be ap.pare nt from the following theoretical considerations.

,Assuming the circuitof Figure 1 to be divided zinto-two parts by broken line A-.-A, theimquency modulation detector circuit.

3 pedance looking into the left-hand portion is given by the expression:

The impedance looking into the right-hand portion is given likewise by the expression:

3 X02 RzXclXtz] R -.7[ C1+ T%2+gm2 z i'z gm2 2 c1 2 X in X 02 The shift in frequency of the Hartley portion of the circuit owing to the presence of resistor I2 is directly attributable to the term RlXnXiz g T il- L in the imaginary part of the expression (1) for ZL. This term is maximum, and the Hartley section experiences maximum frequency shift when R1=XL2. Likewise, for the Colpitts portion of the circuit, the maximum frequency shift will obtain when the term 9 R2XC'1X%'2 i+ i-2 is maximum. This will be the case when R2=Xc2.

Other values of R1 and R2 may, of course, be used if maximum shift is not required. However, as already mentioned, in order that each of the circuits shall continue to be capable of sustaining oscillations, the Values of resistors l2 and I3 must not be made too small. In the instance of the Hartley circuit, the value of resistor I 2 must satisfy the condition:

while, in the instance of the Colpitts circuit, the condition to be satisfied by resistor I3 is:

Values of resistors I 2 and I3 satisfying relations (3) and (4) are such as to make the real parts of the expressions (1) and (2) for Z1. and ZR respectively negative (i. e. to make the resistance of each oscillator circuit negative).

The inclusion of resistor I3 in the cathode circuit of tube 2 makes desirable the connection in shunt therewith of a resonant choke I l tuned to the oscillating frequency of the Colpitts section, to provide a low impedance D.-C. path from the cathode of tube 2 to ground.

It is to be noted also that, although the primary functions of resistors l2 and [3 are to alter the frequencies of the oscillators With which they are respectively associated, each also tends to modify the frequency of the oscillator with which it is not directly associated, and this effect must be taken into account in designing the circuit for oscillation at two specific frequencies.

The circuit above described has numerous applications such as will readily occur to those skilled in the communications art. One such application, which will now be set forth for purposes of illustration, is in a detector of angularvelocity modulated carrier wave signals based on the method described in Patent 2,351,193 of June 13, 1944, to Murray G. Crosby for a. Fre- According to this method two superregenerative detectors are operated at diiferent frequencies respectively above and below the center, or carrier, frequency of the modulated carrier wave signal to be detected. The two detectors are alternately quenched and unquenched to permit oscillations to build up in their tank circuits at a rate determined by the extent of modulation of the carrier wave signal at the time of unquenching. The outputs of the two detectors are combined so as to yield a signal which varies in accordance with the angular-velocity modulation of the input carrier, but which is substantially independent of variations in the amplitude of said carrier. The circuit shown in the patent for applying this method employs separate tubes for the two superregenerative oscillators, two additional tubes comprising a quench controlling oscillator, and yet another tube to vary the tuning of a common tank'circuit used in conjunction with both superregenerative oscillators.

Following this same method, in the circuit of Figure 1 of the present specification, tubes I and 2 may be made to operate alternately as superregenerative detectors at two slightly differing frequencies respectively above and below the carrier frequency of the modulated signal to be detected. A complete embodiment of a circuit for this purpose is shown in Figure 2 where the various components of the circuit of Figure 1 are designated by like reference characters. In this circuit a frequencyor phase-modulated carrier wave signal intercepted by antenna I4 is supplied to tank circuit 3 through inductor l5 coupled to tank circuit inductor 4. A low frequency quench controllin signal generated by oscillator l6, comprising triode ll and tank circuit l8, and which may be of substantially sinusoidal waveform, is supplied in opposite phases, through connections 19 and 20 including condensers 2i and 22, to the grids of tubes l and 2 respectively. The latter are thereby rendered alternately conductive to permit oscillations to build up at the respective frequencies of oscillation of the Hartley and Colpitts circuits in which they are connected. In each circuit oscillations will begin to build up at a time following 'unquenching determined by the extent to which the instantaneous frequency of the modulated carrier wave signal departs from the normal frequency of oscillation of the particular oscillator circuit. In the absence of modulation, the outputs of the two superregenerators will be essentially equal and, when supplied to a conventional push-pull subtractor circuit (not shown), will cancel. However, when the input signal is phaseor frequency-modulated and assuming the frequencies of oscillation of the two oscillator sections to have been displaced by equal amounts respectively above and below the carrier frequency, oscillations will build up to appreciable amplitude in one circuit sooner than in the other during successive alternations of the quench controlling signaL'and subtraction of the two outputs will give rise to a resultant signal indicative of the instantaneous value of modulation.

As shown, the plates of tubes l and 2 are preferably by-passed to ground at their respective frequencies of oscillation by condensers 2-52 and 24, and at quench frequency by series tuned circuits 28 and 29 resonant at the'latter frequency. Detected output is developed across resistors 25 and 26 in the respective plate circuits of tubes I and 2, and is available at terminals El for supply" to a push-pull subtractor circuit (not shown).

For illustrative purposes only, and with no thought of imposing any limitations upon the invention, the following values are given for the components of thecircuit of Figure 2. They are applicable to an'instance in which the carrier frequency of the input signal is 50 megacycles and it is desired to provide a difference between the frequencies of oscillation of the two oscillators of 160 kilocycles (which, however, is not the maXimum obtainable from the circuit) 2 Tubes I and 2 Type 6SN7 Inductor 4 (upper portion) ,c-henry Inductor 4 (lower portion) /3 -henry Condenser 5 l-farads Condenser 6 mi-farads Resistors la and 8a 10,000 ohms Resistors lb and 8b 100,000 ohms Condensers 9 and I0 50 i-farads Choke ll Self resonant at 50 megacycles Resistors l2 and I3 5,000 ohms Tank circuit I8 resonant at 50 kilocycles Condensers 2| and 22 1,000 ,c -farads Condensers 23 and 24 100 i-farads Resistors 2'5 and 26 1,000 ohms Series circuits 28 and 29 resonant at 50 kilocycles It will of course be understood that the invention is susceptible of embodiment in physical forms other than the one here shown, such as will occur to those skilled in the art upon read ing this specification. Accordingly the scope of this invention is to be regarded as subject only to the limitations'imposed by the appended claims.

We claim:

1. In a vacuum tube oscillato'r,a pair of vacuum tubes having at least triode elements, a tank circuit comprising a pair of conjugate reactive branches, a portion of each of said branches being connected in the cathode circuit of one of said tubes and cooperating with said tube to form a separate oscillator adapted to oscillate independently when said other tube is cut off, and means included in the cathode circuit of one of said tubes and operative when said tube is conductive to modify the frequency of oscillation of the oscillator circuit which includes said one tube.

2. In a vacuum tube oscillator, a pair of vacuum tubes having at least triode elements, a tank circuit comprising a pair of conjugate reactive branches, said tank circuit being connected so as to form, in cooperation with each of said tubes, separate oscillator circuits, each adapted to oscillate independently when the other tube is out off and, in each of which, one of said reactive branches cooperates to provide the feedback necessary for oscillation, and means com- .prising a resistor connected in the cathode circuit of one of said tubes for causing said oscillators to oscillate at different frequencies.

3. In a vacuum tube oscillator, a pair of vacuum tubes having at least triode elements, a tank circuit comprising a pair of conjugate reactive branches, said tank circuit being connected so as to form, in cooperation with each of said tubes, separate oscillator circuits, each adapted to oscillate independently when the other tube is cut off and, in each of which, one of said reactive branches cooperates to provide the feed- 6 back necessary for oscillation, and means comprising resistors connected in the cathode circuits of said tubes for causing said oscillators to oscillate at different frequencies.

4. In a vacuum tube oscillator, a pair of vacuum tubes having at least triode elements, a tank circuit comprising an inductance and a capacitance connected in parallel, a resistance connected in parallel with acomponent of said inductance in the cathode circuit of one of said tubes to form an oscillator adapted to oscillate at a predetermined frequency, the produc of said resistance and the mutual conductance of said tube being greater than the ratio of the impedance of said component to the impedance of the remainder of said inductance at said frequency, and a second resistance connected in parallel with a component of said capacitance in the cathode circuit of the other of said tubes to form a second oscillator adapted to oscillate at a predetermined second frequency different from said first frequency, the product of said second resistance and the mutual conductance of said last-named tube being greater than the ratio of the impedance of said component to the remainder of said capacitance at said second frequency.

5. In a vacuum tube oscillator, a pair of vacuum tubes having at least triode elements, a tank circuit comprising an inductance and a capacitance connected in parallel, a resistance connected in parallel with a component of said inductance in the cathode circuit of one of said tubes to form an oscillator adapted to oscillate at a predetermined frequency, said resistance being comparable in magnitude to the impedance of said component inductance at said frequency, and a second resistance connected in parallel with a component of said capacitance in the cathode circuit of the other of said tubes to form a second oscillator adapted to oscillate at a predetermined second frequency different from said first frequency, said resistance being comparable in magnitude to the impedance of said component capacitance at said second frequency.

6. In a vacuum tube oscillator, a pair of vacuum tubes having at least triode elements, a tank circuit comprising a pair of conjugate reactive branches, said tank circuit being connected so as to form, in cooperation with each of said tubes, separate oscillator circuits, each adapted to oscillate independently when the other tube is cut off and, in each of which, one of said reactive branches cooperates to provide the feedback necessary for oscillation, and means comprising substantially equal resistors connected in the cathode circuits of said tubes for causing said oscillators to oscillate at different frequencies.

'7. In a vacuum tube oscillator, a pair of vacuum tubes, each having a plurality of electrodes and. each being adapted to be rendered conductive or non-conductive, a tank circuit reasonant at a predetermined frequency, means coupling said tank circuit to electrodes of one of said tubes to form an oscillatory circuit adapted to oscillate when said one tube is conductive, the frequency of oscillation of said oscillatory circuit being determined by the resonant frequency of said tank circuit, means coupling said tank circuit to electrodes of the other of said tubes to form a second oscillatory circuit adapted to oscillate when said other tube is conductive, the frequency of oscillation of said second oscillatory circuit being determined by the resonant frequency of said tank circuit, and means operative when one of said tubes is conductive to alter the resonant frequency of said tank circuit.

8. In a vacuum tube oscillator, a pair of vacuum tubes, each having at least triode elements and each being adapted to be rendered conductive or non-conductive, a, tank circuit resonant at a predetermined frequency and comprising a' pair of conjugate reactive branches, means coupling said tank circuit to electrodes of one of said tubes to form an oscillator circuit adapted to oscillate when said one tube is conductive, said coupling being made so as to include at least a portion of one of said reactive branches in the cathode circuit of said tube and the frequency of oscilla- 'tion of said oscillatory circuit being determined by the resonant frequency of said tank circuit, means coupling said tank circuit to electrodes of the other of said tubes to form a second oscillatory circuit adapted to oscillate when said other tube is conductive, the frequency of oscillation of said second oscillatory circuit being determined by the resonant frequency of said tank circuit, and means included in the cathode circuit of said first tube and operative when said tube is conductive to alter the resonant frequency of said tank circuit.

9. In a vacuum tube oscillator, a pair of vacuum tubes having at least triode elements, a tank circuit comprising an inductance and a capacitance connected in parallel, a resistance connected in parallel with a component of said inductance in the cathode circuit of one of said tubes to form an oscillator adapted to oscillate at a predetermined frequency, and a second re sistance connected in parallel with a component of said capacitance in the cathode circuit of the other of said tubes to form a second oscillator adapted to oscillate at a predetermined second frequency different from said first frequency.

10. In a vacuum tube oscillator, a pair of vacuum tubes having at least triode elements, a tank circuit comprising a pair of conjugate reactive branches, a resistance connected in parallel with at least a portion of one of said branches in the cathode circuit of one of said tubes to form an oscillator adapted to oscillate at a predetermined frequency, and a second resistance connected in parallel with at least a portion of the other of said branches in the cathode circuit of the other of said tubes to form a second oscillator adapted to oscillate at a predetermined second frequency different from said first frequency,

11. In a Vacuum tube oscillator, a pair of vacuum tubes each having at least triode elements, a tank circuit comprising an inductance and a capacitance connected in parallel, means coupling said tank circuit to one of said tubes to form a Colpitts circuit adapted to oscillate at a predetermined frequency, and means coupling said tank circuit to said other tube to form a Hartley circuit adapted to oscillate at a different frequency.

12. In a vacuum tube oscillator, a pair of vacuum tubes having at least triode elements, a tank circuit comprising an inductance and a capacitance connected in parallel, a resistance connected in parallel with a component of said inductance in the cathode circuit of one of said tubes to form an oscillator adapted to oscillate at a predetermined frequency, a resistance connected in parialiel with a component of said capacitance in the cathode circuit of said other tube to form a second oscillator adapted to oscillate at a second frequency, and a choke substantially resonant at said second frequency connected in shunt with said last-named resistance and said component of said capacitance.

WILLIAM E. BRADLEY. JOSEPH C. TELLIER.

No references cited.

Certificate of Correction Patent No; 2,469,811. May 10, 1949.

WILLIAM E. BRADLEY ET AL. It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 5, Equation (1) for X read X lines 13 and 14, Equation (2) at the end thereof, insert a closing parenthesis;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 11th day of October, A. D. 1949.

THOMAS F. MURPHY,

Auiatant-Uommiadmf of Patents. 

